The generation of neutrophils from hematopoietic precursors and their release to the peripheral circulation are highly regulated processes that ensure the maintenance of homeostatic neutrophil levels in the blood and their rise in response to bacterial infections and other signals. Defective neutrophil maturation/release are associated with various forms of neutropenia, which may precede and be pathogenetically linked to the development of myeloid leukemias. G-CSF has emerged a critical physiological regulator of granulopoiesis since mice carrying homozygous deletions of colony-stimulating factor (G-CSF) or its receptor are severely neutropenic, and dominant-negative mutations of G-CSFR have been linked to severe defects of granulopoiesis. Administration of G-CSF induces an expansion of myeloid lineage cells in the bone marrow, and promotes the release of neutrophils and hematopoietic progenitor cells from the bone marrow to the peripheral blood. Based on these properties, G-CSF is widely used to induce granulopoiesis and to mobilize hematopoietic progenitors to the peripheral blood. More recently, a CXCR4 competitive inhibitor, AMD3100/Plerixafluor, has been approved by FDA and a mobilizing agent for hematopoitic precursors in conjunction with G-CSF. The biological activities of G-CSF are solely mediated by its activation of the G-CSF-receptor (R) that is expressed on myeloid lineage progenitor cells. Compelling evidence from genetic studies and other studies demonstrated that G-CSF indirectly promotes hematopoietic cell and neutrophil mobilization to the peripheral blood by modulating the activities of the chemokine SDF1 and/or its receptor CXCR4. WHIM, a genetic disorder associated with mutations in the intracellular domain of CXCR4 leading to increased CXCR4 function causes a retention of immature neutrophils into the bone marrow and severe peripheral neutropenia. AMD3100, a competitive inhibitor of SDF-1 binding to its receptor and a mutant form of SDF-1, which induces prolonged downregulation of the CXCR4 surface receptor, promotes the mobilization of neutrophils and hematopoietic cells to the peripheral blood. During stem cell mobilization with G-CSF, SDF-1 and CXCR4 protein levels decrease in the bone marrow. We have examined the mechanisms responsible for reduced CXCR4 expression. Initially, we found that G-CSF reduces CXCR4 expression in bone marrow Gr1+ myeloid cells, which express G-CSFR. In additional studies, we have obtained evidence that the transcriptional repressor Gfi-1 is involved in G-CSF-induced mobilization of granulocytic lineage cells from the bone marrow to the peripheral blood: G-CSF promotes expression of Gfi-1, which reduces CXCR4 expression and function. In related experiments, we have generated mutants of CXCR4 that mimic mutations in the C-terminal domain found in patients with WHIM syndrome. We have examined the signaling mechanisms from wild-type CXCR4 and compared with signaling from mutants CXCR4 receptors. Our results indicate that unlike the normal receptor, mutant CXCR4 fails to appropriately recruit beta arrestin2 to the receptor complex. As a consequence internalization of the mutant CXCR4 receptor from the cell surface to the cytoplasmic compartment is delayed, degradation is delayed, and signaling from the mutant receptor is also delayed. Since WHIM patients are heterozygotes for the mutant CXCR4 receptor and carry both the normal and the mutant allele, the net result is that CXCR4 signaling is extended in time, as it is the result of activation of both the normal and the mutant receptor. Thus, patients with WHIM have a super-functional CXCR4 receptor and presumably fail to release neutrophils from the bone marrow to the peripheral blood due to continuous signaling by the ligand SDF1, which holds the mature neutrophils in the bone marrow compartment. In addition to promoting the release of mature myeloid cells, G-CSF promotes the release of HSPC (hematopoietic stem/progenitor cells) from the bone marrow to the peripheral blood. Mobilization of hematopoietic progenitor cells (HPC) from the bone marrow to the peripheral blood by G-CSF is the primary means to acquire stem cell grafts for hematopoietic cell transplantation avoiding invasive bone marrow collection. Since HPC represent a small minority of all blood cells mobilized by G-CSF, there is a need for understanding the underlying mechanisms to develop selective drugs. We now found that G-CSF indirectly reduces expression of surface vascular cell adhesion molecule 1 (VCAM-1) on bone marrow HSPC, stromal cells and endothelial cells by promoting the accumulation of microRNA-126 (miR126)-containing microvescicles/exosomes in the bone marrow extracellular compartment. We find that HSPC, stromal cells and endothelial cells readily incorporate these exosomes, and that miR126 represses VCAM-1 expression on bone marrow HSPC, stromal cells and endothelial cells. In line with this, miR126-null mice display a reduced mobilization response to G-CSF. Altogether, our results implicate miR126 in the regulation of HPC trafficking between the bone marrow and peripheral sites, clarify the role of VCAM-1 in G-CSF-mediated mobilization, and have important implications for improved approaches to selective mobilization of HPC. Ongoing studies designed to further understanding of HSPC mobilization have detected an important role of the receptor/ligand pair EphrinB2/EphB4. Thus, blocking this interaction with blocking peptides prevents stem cell mobilization from the bone marrow to the blood in mice. Immunohistochemical analysis of bone marrow sections has revealed that bone marrow sinusoidal endothelium selectively expresses EphB4, which is not detected in hematopoietic cells. The sinusoidal endothelium does not express ephrinB2, which is instead expressed in the bone marrow hematopoietic cells. Transmigration experiments suggest that EphrinB2/EphB4 are critical for HSPC trans-endothelial migration, but not the migration of other mature hematopoietic cells. Thus, agonist activation of ephrinB2 should promote selective HSPC exit from the bone marrow. Conversely, inhibition of EphrinB2/EphB4 interaction, which blocks HSPC exit from the bone marrow, could be exploited to reduce HSPC contribution to tumor progression. Other ongoing studies have focused on the generation of hematopoietic cells from aortic endothelium, and the characterization of the biochemical requirements underlying this critical developmental step. We have discovered that EphrinB2 expression is critical to the development of hematopoiesis from murine ES cells, and that EphrinB2 critically regulates the expression of transcription factors that orchestrate developmental hematopoiesis. We are currently exploring the mechanistic aspects of EphrinB2 expression in ES cells at bhe very early stages of differentiation. There is a critical need for generation of HSPC to be used in clinical transplantation. The ultimate goal of our investigation is to develop a method for production of such cells in vitro.